(a) Field of the Invention
The present invention relates to a method for fabricating metal wirings of a semiconductor device and, in particular, to a method for fabricating the metal wirings of the semiconductor device having a damascene structure.
(b) Description of the Related Art
Recently, as semiconductor devices have been highly integrated and process techniques have been enhanced, conventional aluminum wirings are being replaced by copper wirings for improving the device characteristics such as operation speed and resistance of the device, and parasitic capacitance between the wirings.
However, since copper has very poor etching characteristics, a damascene process replaces the conventional etching process.
In the damascene process, a dual damascene pattern having trenches for forming via holes and wirings in an inter metal dielectric is formed, and then a copper layer is deposited thickly enough to fill the contact holes. After forming the dual damascene pattern, an annealing process is performed on the copper layer so as to remove the impurities that are included during the deposition of the copper layer. The polishing process is carried out on the upper surface of the inter metal dielectric using a chemical mechanical polishing technique, thereby forming the metal wirings and plugs.
Because the via holes and trenches are filled with metal, the edges of the via holes and the trenches are formed with a rounded shape so as to increase the step coverage.
However, during the etching process for forming the via holes and the trenches, inner walls of the trenches develop a striated profile due to the reflectivity difference between the inter metal dielectric layer and the etch stop layer. Such striations deteriorate the step coverage of the metal layer formed inside the trench.
Also, the striations generate a gap between a thin film deposited on the inner walls of the trench and the surface of the trench, which causes problems during the planarization process, resulting in cracking of the thin film. Such defects are common in regions where the metal wirings are crowded.